1. Field of the Invention
The present invention relates to a semiconductor device having a circuit that comprises a thin film transistor (hereinafter referred to as TFT). For instance, the invention relates to the structure of an electrooptical device represented by a liquid crystal display and an EL display, and of an electronic equipment equipped with such electrooptical device as one of its parts. The term ‘semiconductor device’ in this specification designates devices in general which utilize semiconductor characteristics to function and, therefore, the electrooptical device, the semiconductor circuit and the electronic equipment all fall into the range of the semiconductor device.
2. Description of the Related Art
Application of TFTs (Thin Film Transistors) to active matrix liquid crystal displays (hereinafter referred to as AM-LCDs) has actively been developed because TFTs allow of the use of inexpensive substrates on which they are to be formed. Among TFTs, the one that utilizes a crystalline semiconductor film (typically, a polysilicon film) can provide a high mobility and hence is considered to be suitable for integrating functional circuits on the same substrate to realize image display of high definition.
Basically, an AM-LCD comprises a pixel portion (a region where a plurality of pixels are arranged) for displaying an image and a driver circuit portion for driving a TFT of each of the pixels arranged in the pixel portion, the portions formed on the same substrate. The driver circuit portion may be divided into a gate line driver circuit (gate driver circuit) driving a gate line and a source line driver circuit (source driver circuit) transmitting a video signal to each TFT.
In the System on Panel, proposed lately, a logical circuit portion including a signal divider circuit, γ correction circuit is formed on the same substrate other than the pixel portion and the driver circuit (also called as a peripheral driver circuit) portion.
However, capabilities required for TFTs are different between the pixel portion and the driver circuit portion, thereby making it difficult to meet all ways with one TFT structure. In other words, the TFT structure in which a TFT for forming a driver circuit such as a shift resister circuit or a latch circuit where the importance is attached to high speed operation is compatible with a TFT for forming a buffer circuit, a sampling circuit, or a pixel circuit that gives importance to high withstand voltage characteristic has not been established yet under the present circumstances.